The term “ionic liquid” refers to a salt composed solely of ions which generally has a melting point of 100° C. or below. Ionic liquids have attracted attention because of their outstanding characteristics such as non-volatility, flame retardance and high ionic conductivity, and also because their properties and functions can be variously designed. On account of such qualities, ionic liquids are regarded as promising for such applications as solvents in green chemistry that is gentle on the environment, and as electrolytes in electrical storage devices.
However, because most hitherto known ionic liquids include halogen atoms such as fluorine atoms on the anion, there remain problems with these from the standpoint of their environmental impact. Moreover, halogen-free ionic liquids are known to be inferior to ordinary fluorine-containing anion-containing ionic liquids in terms of their physical properties; for example, they have a poor heat resistance and a high viscosity.
In order for ionic liquids to be used in electrochemical applications such as electrolytes for electrical storage devices, they are required to have electrochemical properties such as a high ionic conductivity and a wide potential window. Such ionic liquids have hitherto been reported, although these have halogen atoms, and especially fluorine atoms, on the molecule. Ionic liquids which are halogen-free and also have a wide potential window have not hitherto been known.